In a typical scanned display system, a point of illumination is scanned in two dimensions to form a rasterized image. Typically, one scan axis (fast-scan axis) is scanned at an integer multiple of the other axis (slow-scan axis). Both axes are typically scanned with a unidirectional ramp or sawtooth function having an active video portion in which the point of illumination constructs the image and a “flyback” time during which illumination is disabled (i.e. blanked). The resulting fast-scan lines are all parallel to each other and this ensures a very uniform spatial resolution.
Some systems contain inertia that limits the frequency of the scan function in the fast-scan axis. Use of a sinusoidal scan function rather than a ramp allows the scan frequency to be increased. In this case, the image can be scanned bidirectionally (e.g., both left-to-right and right-to-left). Use of the sinusoidal scan function eliminates the need to “flyback” in the fast-scan axis which reduces or eliminates the blanking time.
FIG. 1 shows a scan trajectory having a sinusoidal component on the fast-scan axis (horizontal axis) and a sawtooth component on the slow-scan axis (vertical axis). Scan trajectory 100 is shown superimposed upon a grid 102. Grid 102 represents rows and columns of pixels that make up a display image. The rows of pixels are aligned with the horizontal dashed lines, and columns of pixels are aligned with the vertical dashed lines. The image is made up of pixels that occur at the intersections of dashed lines. On scan trajectory 100, the beam sweeps back and forth left-to-right in a sinusoidal pattern, and sweeps vertically (top-to-bottom) in a sawtooth pattern with the display blanked during flyback (bottom-to-top).
As shown in FIG. 1, the vertical sweep rate is typically set such that the number of horizontal sweeps equals the number of rows in the grid, and the vertical scan position at any time is approximated as a corresponding row. For example, as shown in FIG. 1, each horizontal sweep 110 from left-to-right corresponds to one row 112 and the following sweep from right-to-left 120 may correspond to the next row 122. In the displayed image, however, the horizontal fast-scan lines are not parallel to each other resulting in a non-uniform spatial resolution and the resulting image quality is degraded—especially at the extremes of the fast-scan axis. This image artifact is referred to as “raster pinch”. Raster pinch is shown in FIG. 1 where pixels 134 and 138 are more closely spaced (“pinched”) than pixels 138 and 144. Raster pinch is also shown where pixels 144 and 148 are pinched.
FIG. 2 shows prior art beam deflection waveforms that result in the scan trajectory of FIG. 1. Vertical deflection waveform 210 is a sawtooth waveform, and horizontal deflection waveform 260 is a sinusoidal waveform. Horizontal deflection waveform is a sinusoid having period TH. Vertical deflection waveform 210 is a sawtooth waveform having period TV which is an integer multiple of TH. The sawtooth vertical deflection waveform 210 includes a rising portion corresponding to the sweep of trajectory 100 from top-to-bottom, and also includes a falling portion corresponding to the flyback from bottom-to-top. After the flyback, the vertical sweep traverses substantially the same path on each trajectory.
Blanking waveform 280 is also shown in FIG. 2. The scanned beam is blanked (no pixels are displayed) during flyback, and is not blanked during the vertical sweep. For clarity, the flyback of the scanned beam is not shown in FIG. 1.